A number of spinal stabilization systems are currently available for use in the lumbar spine to provide stabilization of the spine during spinal fusion. Many stabilization systems include some type of screw implant, which usually consists of a bone screw and a receiver body attached to the screw. The screw is designed to be inserted into a vertebral body or other bone structure to be stabilized. The receiver body is designed to receive an elongated fixation element, such as a rod. Typically, the receiver body has a transverse passage formed by slots or openings for receiving the rod. The rod is inserted into the transverse passages of two or more screw implants, forming a rigid bridge between the implants. The rod may be introduced using a “captured” technique or a freehand technique. In the captured technique, a guidance device with a defined range of motion controls the path of the rod as it is introduced into the implants. In the freehand technique, the rod is inserted and maneuvered into the implant by hand. Once the rod is inserted into the implants, set screws or other locking elements are used to lock down the rod in the receiver bodies. The rigid bridge construct is used to stabilize adjacent vertebrae and promote bone fusion.
In some procedures, the screw implants and fixation rod are implanted in an open surgical method, where the skin of the patent is incised from the cranial aspect of the area to the caudal aspect. Open incisions of this type occasionally result in significant incision length. Of greatest concern, when examining the open technique, is the trauma to the muscles, nerves, and other soft tissue of the back. This trauma results in biomechanical instability, greater possible necrosis, and an increased time for recovery. Trauma can be reduced by using minimally invasive surgery (MIS), which involves the use of smaller incisions, and muscle splitting rather than cutting. Percutaneous surgery is one form of MIS that utilizes very small stab incisions for the introduction of the screw into the patient. In percutaneous surgery, the screw implants and/or rods are inserted into the patient through small tubes sometimes referred to as downtubes or extension tubes. Extension tubes can be connected to the screw implants to serve as temporary extensions of the screw body. The extensions provide a conduit through the surface of the patient's skin, and provide access to the surgical site through a very small opening. Extension tubes are disconnected from the screw implants and removed from the patient once the surgery is complete.
MIS screw systems offer the advantage of reducing trauma to the patient by minimizing the size of incisions. Nevertheless, the small incisions create a major challenge. MIS screw implants are difficult to see after implantation because the implants are covered by skin and tissue above implants. This makes it difficult or impossible for the surgeon to see the transverse passage of the implant and the rod during rod insertion. The relative position of the rod can be monitored using fluoroscopy. But even where fluoroscopy is used, depth perception is very limited when watching the image. As such, it is difficult to determine whether the rod is passing through the transverse passage of the receiver body as intended. In some instances, the rod may appear to be passing through the receiver body, when in fact it is passing in front of or behind the receiver body. It is not uncommon to find that the rod has passed along the outside edge of the receiver body, necessitating the total removal and replacement of the rod.
Even when imaging techniques are available, there is still a need for better ways to visualize the introduction of the rod into the receiver body, so that the proper placement of the rod can be confirmed.